Gilles Chanteperdrix wrote:
> On Jan 17, 2008 11:42 AM, Jan Kiszka <[EMAIL PROTECTED]> wrote:
>> Gilles Chanteperdrix wrote:
>>> Hi,
>>>
>>> after some (unsuccessful) time trying to instrument the code in a way
>>> that does not change the latency results completely, I found the
>>> reason for the high latency with latency -t 1 and latency -t 2 on ARM.
>>> So, here comes an update on this issue. The culprit is the user-space
>>> context switch, which flushes the processor cache with the nklock
>>> locked, irqs off.
>>>
>>> There are two things we could do:
>>> - arrange for the ARM cache flush to happen with the nklock unlocked
>>> and irqs enabled. This will improve interrupt latency (latency -t 2)
>>> but obviously not scheduling latency (latency -t 1). If we go that
>>> way, there are several problems we should solve:
>>>
>>> we do not want interrupt handlers to reenter xnpod_schedule(), for
>>> this we can use the XNLOCK bit, set on whatever is
>>> xnpod_current_thread() when the cache flush occurs
>>>
>>> since the interrupt handler may modify the rescheduling bits, we need
>>> to test these bits in xnpod_schedule() epilogue and restart
>>> xnpod_schedule() if need be
>>>
>>> we do not want xnpod_delete_thread() to delete one of the two threads
>>> involved in the context switch, for this the only solution I found is
>>> to add a bit to the thread mask meaning that the thread is currently
>>> switching, and to (re)test the XNZOMBIE bit in xnpod_schedule epilogue
>>> to delete whatever thread was marked for deletion
>>>
>>> in case of migration with xnpod_migrate_thread, we do not want
>>> xnpod_schedule() on the target CPU to switch to the migrated thread
>>> before the context switch on the source CPU is finished, for this we
>>> can avoid setting the resched bit in xnpod_migrate_thread(), detect
>>> the condition in xnpod_schedule() epilogue and set the rescheduling
>>> bits so that xnpod_schedule is restarted and send the IPI to the
>>> target CPU.
>>>
>>> - avoid using user-space real-time tasks when running latency
>>> kernel-space benches, i.e. at least in the latency -t 1 and latency -t
>>> 2 case. This means that we should change the timerbench driver. There
>>> are at least two ways of doing this:
>>> use an rt_pipe
>>>  modify the timerbench driver to implement only the nrt ioctl, using
>>> vanilla linux services such as wait_event and wake_up.
>> [As you reminded me of this unanswered question:]
>> One may consider adding further modes _besides_ current kernel tests
>> that do not rely on RTDM & native userland support (e.g. when
>> CONFIG_XENO_OPT_PERVASIVE is disabled). But the current tests are valid
>> scenarios as well that must not be killed by such a change.
> 
> I think the current test scenario for latency -t 1 and latency -t 2
> are a bit misleading: they measure kernel-space latencies in presence
> of user-space real-time tasks. When one runs latency -t 1 or latency
> -t 2, one would expect that there are only kernel-space real-time
> tasks.

If they are misleading, depends on your perspective. In fact, they are
measuring in-kernel scenarios over the standard Xenomai setup, which
includes userland RT task activity these day. Those scenarios are mainly
targeting driver use cases, not pure kernel-space applications.

But I agree that, for !CONFIG_XENO_OPT_PERVASIVE-like scenarios, we
would benefit from an additional set of test cases.

Jan
-- 
Siemens AG, Corporate Technology, CT SE 2
Corporate Competence Center Embedded Linux

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